Among aromatic vinyl compounds, styrene especially is an industrially very important compound as a raw material for preparing polystyrene, a synthetic rubber, an ABS resin, etc. and has been industrially manufactured in a large amount.
In general, an aromatic vinyl compound is extremely susceptible to polymerization so that it easily undergoes polymerization under the influence of the heat generated during its preparation or purification step, thereby causing a lowering in the yield of the monomeric aromatic vinyl compound and, further, the formation of fouling in the facilities involved to induce trouble in operating the facilities. As the countermeasure taken hitherto, a process wherein a certain polymerization inhibitor is added to the process fluid containing an aromatic vinyl compound has been proposed and practically carried out. For example, there have been proposed a process employing phenols, nitrosophenols and/or nitrophenols (Japanese Patent Application Laid-Open No. Sho 63-316745), one employing piperidine-1-oxyls (Japanese Patent Application Laid-Open No. Hei 1-165534), one employing nitrophenols together with piperidineoxyls (Japanese Patent Application Laid-Open No. Hei 6-166636), one employing an alkylsulfonic acid and dialkylhydroxylamine in combination (U.S. Pat. No. 4,654,450 specification), one employing phosphoric acid ester and an alkylsulfonic acid in combination (U.S. Pat. No. 4,425,223 specification), one adding phenol-, amine- and/or nitroso-related polymerization inhibitors together with a sulfonic acid compound such as dodecylbenzene-sulfonic acid or the like and its salt in order to suppress an increase in viscosity in an olefin preparation process (Japanese Patent Application Laid-Open No. Hei 7-166152), and one adding a quinone-, hydroquinone- or nitroso-related polymerization inhibitor, or an amine-related polymerization inhibitor such as phenylenediamine together with a sulfonic acid compound such as dodecylbenzenesulfonic acid or the like and its salt for preventing formation of fouling in step treating a vinyl compound (Japanese Patent Application Laid-Open No. Hei 8-34748). Among them, especially 2,4-dinitrophenol (DNP) and 2,4-dinitro-6-methylphenol, 2,4-dinitro-6-sec-butylphenol (DNBP) which are nitrophenol compounds have been often used as the polymerization inhibitors for the aromatic vinyl compound. However, these chemical agents have a characteristic property that their inhibitory power toward the polymerization reaction decreases gradually and strict care is needed in handling them because they belong to toxic substances. Accordingly, there has been strongly desired a process which is capable of maintaining a polymerization-inhibiting effect by these chemical agents and reducing the amount of them used.